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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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44                                 ALASKAN GLACIER STUDIES
most of its area it could not maintain its size and form. Back of the fringe of ablation moraine, which checks ablation, there would, of necessity, be a depressed area due to wasting away of the clear ice unless new supplies were being constantly added.
The surface of the Malaspina Glacier, is, in general, a vast expanse of clean ice (PI. IX. B); but the monotony of the wa^te of ice is relieved by areas of moraine, especially prominent in two positions, as ablation moraine around the periphery (PI. X, A) and as medial moraines between lobes. The latter areas represent lateral moraines of the glaciers forming the lobes, brought in relief by ablation. The former, consisting mainly of material incorporated in the ice in the mountain valleys and the glacier reservoirs, is concentrated at the surface in broad tracts, and in places to considerable depth, by the ablation of the ice through which the debris was scattered. Possibly some of this peripheral moraine is supplied by uprising bottom layers, but of this we have no evidence. A third class of moraine, more difficult to understand, is long, sinuous lines of scroll-like moraine, forming a black tracing in the expanse of clear white ice. Russell suggested as explanation of such moraines some unknown variations in ice currents and this is possibly a correct interpretation tho ugh it is difficult to conceive the process. Another hypothesis suggests itself to us, namely, that these sinuous moraines may, in part at least, represent avalanche falls upon the upper glaciers, spread out in lines by the currents of the ice. We have seen them only from a distance, but they are most striving phenomena, resembling the lines of foam that develop in a swirling river current.
The periphery of the Malaspina Glacier is for the most part skirted by a fringe of alluvial fan deposits, crossed by a multitude of streams of which the largest, all raging torrents, are the Kwik, Kame, Osar, Manby, Yahna, Fountain, and Yahtse (PI. VIII). All, burdened with vast quantities of clay, sand, and gravel, are depositing rapidly in the sea and thus extending the area of the fringe, and, by deposit in their beds, being forced to divide and subdivide, and constantly shift their courses, are raising the surface of the alluvial fringe. Where the streams cross the narrow strip of outwash gravel plain the rapid deposit by the ever-shifting branches prevents the growth of vegetation and, therefore, gives rise to barren alluvial fans; but in those portions where only small streams flow there is a growth of vegetation, in places spruce and hemlock forest, elsewhere cotton-wood, or only alder and willow. -Because of the fact that the plain lies near sea level, while the ice immediately back of it is so covered by moraine that it has little effect on the climate, the forest here grows as luxuriantly as elsewhere on this part of the Alaskan coast wherever the absence of glacial streams gives sufficient freedom from destructive inundation. This forest grows not only in front of the glacier, but, wherever there is a sufficient degree of stagnation of the ice to permit the growth of trees without the undermining action of a slumping soil, it encroaches on the glacier itself (PI. X, B). This is not a general condition around the whole margin of the Malaspina but is confined to two limited regions of exceptional stagnation,—along and near the Kwik River, and in the neighborhood of the Yahtse River.
Malaspina Glacier is interesting in itself as a perfect type of a kind of glacier not recognized until Russell described its characteristics. It has an even wider interest from the fact that it is an existing example of a type which in former times must have been far more extensive. During the period of former expansion of Alaskan glaciers it was duplicated in many places along the Alaskan coast. The piedmont type of glacier was repre-